The developing fetal lung must have a supply of long chain saturated fatty acids to synthesize pulmonary surfactant. Absence of pulmonary surfactant at birth results in the Respiratory Distress Syndrome, a common, often fatal illness of premature infants. In addition to serving as a required substrate for surfactant, fatty acids may augment surfactant phosphatidylcholine synthesis by stimulating the activation of a key regulatory enzyme. Among the important sources of fatty acids available to fetal lung, de novo synthesis within lung is likely since it increases during gestation parallel to phosphatidylcholine synthesis. Utilization of fatty acids by any tissue is probably modulated by binding to an intracellular protein, fatty acid binding protein (FABP). FABP may compartmentalize intracellular fatty acids and have direct effects on enzymes of fatty acid utilization. This proposal seeks to answer three questions about de novo fatty acid synthesis in fetal lung: 1) Is de novo fatty acid synthesis induced by dexamethasone and/or thyroid hormone, 2) do fatty acids or fatty acid synthesis augment phosphatidylcholine synthesis, and 3) what are the binding characteristics of FABP in lung? A fetal rabbit explant model will be used to answer the first two questions. The lung explants will be cultured with dexamethasone and/or thyroid hormone to examine effects on fatty acid synthesis, the distribution of fatty acids among lung lipid species, and the activity of relevant enzymes. The role of cellular fatty acids in stimulating phosphatidylcholine synthesis will be examined by focusing on the activity of cholinephosphate cytidylyltransferase, including its precursor-product concentrations and specific activities. The availability of exogenous fatty acids and the stimulation or inhibition of de novo fatty acid synthesis are the major independent variables to be tested. Pulmonary FABP will be separated by gel chromatography and ion exchange chromatography. The binding characteristics of FABP will be determined first in a pure population of differentiated type II pneumocytes and then in fetal lung. Fatty acid binding affinity, capacity and differential binding of fatty acids based on saturation and source will be determined. The effects of FABP on enzymes of lung phospholipid synthesis will be determined and a developmental profile of FABP will be sought.